The production of polymer powder by polymerisation reactions of monomers in the presence of catalysts is well-known. For example, processes are known and widely operated commercially using both fluidised bed reactors and slurry phase reactors.
In a slurry polymerisation process the polymerisation is conducted in a stirred tank or, preferably, a continuous loop reactor in which a slurry of polymer particles in a liquid medium comprising hydrocarbon diluent is circulated. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of monomer and polymer product is removed from the reactor by removing a portion of the slurry.
In a single reactor system the slurry withdrawn from the reactor is treated to separate the polymer particles from the hydrocarbon diluent and other components, such as unreacted monomers, which it is generally desired are recycled to the reactor.
The process where polymer is formed in two reactors in series is also known. The separate reactors can be operated to produce the same product in each reactor, but are most advantageously operated to produce different products in each reactor, in particular to make bimodal polymer products.
In such a process, polymer is produced in the first reactor, withdrawn in the form of a slurry and passed to a second reactor where further production of polymer takes place. Polymer slurry withdrawn from the subsequent reactor is treated to separate the polymer solids from diluent and unreacted reactants, which it is generally desired to recycle to the process.
In a typical separations process, which is also generally used for single reactor systems, withdrawn slurry is heated and passed to a first separation step in which the majority of the diluent and unreacted monomers and comonomers are separated from the polymer solids as a gas (flash gas) at relatively high pressure such that the gas can be condensed without compression and recycled. This is commonly referred to as a “flash step”.
Remaining solids and residual diluent are then sent to a second separation step, which may be a further flash tank or may be a flush column where the solids are contacted with a flush gas, such as nitrogen, to remove residual diluent, monomers and comonomers. The second separation step is usually at a lower pressure, and diluent, unreacted monomer and any comonomer separated in the second separator need to be separated from any flush gas, and usually need to be compressed prior to recycle.
Thus, the overall polymerisation process generally includes both high pressure and low pressure recovery systems for recovery and recycle of diluent, monomers and comonomers.
The polymer solids may be taken to further processing, such as blending or pelleting, or to storage.
WO 2006/015807 discloses a polymerisation process in which the gas stream recovered from flashing diluent and unreacted monomer from a slurry of polymer solids is passed to a fractionator. The application of the fractionator to a process comprising two polymerisation loop reactors operated in series is also described. In particular with respect to FIG. 3 there is described a process where the fractionator is used to treat both the flash gas from the second reactor in series and also gas separated from the effluent between the two reactors.